CRT's are well known in the art and include a plurality of electron beam forming elements (electron gun) situated in the neck of an evacuated envelope, generally made of glass. The electron beam is directed to a phosphor viewing screen formed on the inner surface of an enlarged funnel shaped portion of the envelope connected to the neck. The electron gun elements are cup-shaped grid electrodes positioned in close proximity to each other and to an electrically heated cathode that has been treated to enhance its ability to give off electrons. Some of the electrodes in the electron gun serve to focus and accelerate the electron beam. A control electrode affects the intensity or modulation of the beam in accordance with an input video signal. The CRT is supplied with a high voltage, on the order of 12 KV for small monochrome CRT's, which is applied to an electron permeable conductive coating applied over the phosphor screen and to an inner conductive coating on the funnel portion of the tube envelope. The high voltage, which is also internally supplied to one or more of the electrodes in the electron gun, is brought into the envelope through an anode button sealed into the tube wall. The outer surface of the envelope is coated with a conductive material commonly referred to as "dag" coating. This outer coating is connected to ground. Thus, not only is the high voltage supplied to the CRT, but the glass envelope of the CRT functions as the dielectric of a capacitor having plates formed by the inner conductive coating and the outer "dag" coating. This high voltage capacitor is useful for filtering the output of the high voltage power supply.
As is well known, CRT manufacture is a relatively complex undertaking. One major task is to assure that the elements used in fabrication are free of contaminants and minute dirt particles. A "dirty" tube is prone to internal arcing, especially in the areas of the closely spaced gun electrodes. The resultant heavy discharges of electrical energy between the high voltage electrodes in the tube and the neighboring lower voltage electrodes can be very destructive, not only to the tube elements, but also to the associated circuitry. While precautions are taken during manufacture to eliminate such particles, including a technique known as "spot knocking" involving vaporizing contaminant particles with high voltage energy, occasional arcs usually occur during normal tube operation. It is imperative that precautions be taken to protect the cathode and heater gun electrodes and their surrounding low impedance circuits from the effects of such arcs.
While the high voltage is coupled through the wall of the CRT, the lower DC voltages and signal voltages are supplied to connection pins that are embedded in the base of the tube neck. It is also well known to provide arc protection in the base socket that connects the external circuitry to the CRT pins. Many forms of arc protection are used in the prior art, a common one being air gaps located between the electrodes of the CRT and a ground connection. It is also well known to mount the CRT socket on a printed circuit board which may carry some of the resistors that are connected between the CRT pins and their supply circuits. The printed circuit board has also been used to define the spark gaps with the CRT socket connectors being terminated to the printed circuit board and to conductive foil strips interconnecting the terminations with a ground connection, generally the picture tube dag coating. The spark gaps are formed in the foil strips by physically removing portions of the printed circuit board and foil thereby breaking the connection between the socket connectors and ground. Despite such precautions, it has been observed that a large number of arcs occur to the cathode and heater electrodes which, because of their relatively remote position from the high voltage electrodes in the electron gun structure, should be considered least susceptible to arcs.
Applicant has determined that the cathode and heater structures rarely receive "primary" arcs, but rather are subjected to "secondary" arcs, that is arcs created by failure to discharge the primary arc energy quickly enough.